The invention relates to nematic liquid-crystal compositions based on bisalkenyl compounds and terminally fluorinated compounds. These compositions are particulary suitable for plasma-addressed display devices.
Plasma-addressed displays (PADs) are of great utility for high-information displays, which are of commercial interest. Such PADs are used in TV applications and in high-information displays for computer screens, automobiles and aircraft.
PADs have electrical switching elements and a plasma cell with a plurality of addressing channels which is connected to the display cell.
Such PADs are disclosed, for example, in WO 96/00925, EP 0 628 944, EP 0 545 569, U.S. Pat. No. 4,896,149 and U.S. Pat. No. 5,077,553.
In a PAD, the switching elements are addressed via a multiplexing scheme. This charges the electrodes of a pixel in the limited time during which they are active. They subsequently become and remain inactive until they are addressed again in the next cycle. Consequently, the voltage change at a plasma-addressed pixel is an unwanted, but very crucial characteristic of such a display. Discharge of the electrodes of the pixel is determined by two factors, the capacity of the pixel and the specific resistance of the liquid-crystal material, i.e., the liquid crystal, between the electrodes.
Conventional liquid-crystalline materials for active matrix displays (AMDs) are unsuitable for PADs since they have excessively high values for the dielectric anisotropy.
There therefore continues to be a great demand for liquid-crystal compositions having high specific resistance and having other material properties which are suitable for use in PADs, such as, for example, a broad nematic mesophase range with an extremely low smectic-nematic transition temperature and the absence of crystallization at low temperature.
A further requirement in such compositions is for high steepness of the characteristic lines (small difference between V90 and V10).
An object of the invention is to provide liquid-crystal compositions which have very high specific resistance and which also satisfy the other requirements.
An additional requirement is for compositions having low xcex94n values of  less than 0.1 and rotational viscosities of xe2x89xa6150 mPaxc2x7s at 20xc2x0 C., preferably xe2x89xa6120 mPaxc2x7s, in particular xe2x89xa6100 mPaxc2x7s.
It has now been found that nematic liquid-crystal compositions for plasma addressed liquid crystal displays (PALCDs) comprising at least one compound of the formula I 
in which
R1 and R2, independently of one another, are alkenyl or alkenyloxy having 1 to 8 carbon atoms,
each Z1 and Z2, independently of one another, and if Z1 is present a number of times, these radicals also independently of one another, are a single bond, xe2x80x94(CH2)2xe2x80x94 xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, trans-CHxe2x95x90CHxe2x80x94, xe2x80x94CH2Oxe2x80x94 or xe2x80x94OCH2xe2x80x94, where Z2 is preferably a single bond,
each A1, A2 and A3, independently of one another, and if A1 is present a number of times, these radicals also independently of one another, are trans-1,4-cyclohexylene, 1,4-cyclohexenylene, in which also one or two nonadjacent CH2 groups may be replaced by oxygen, 1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene, 2,6-difluoro-1,4-phenylene or 1,5-pyridyl or 1,5-pyrimidyl, in which one or two H atoms are optionally replaced by F,
where at least one of the rings A2 is preferably a trans-1,4-cyclohexylene rings, and particularly preferably both rings A2 and A3 are trans-1,4-cyclohexylene, and
n is 0, 1 or 2,
are particularly suitable for PAD applications. Z2 is preferably a single bond, and at least one of the rings A2 and A3 is preferably a trans-1,4-cyclohexylene ring. Particularly preferably, both rings A2 and A3 are trans-1,4-cyclohexylene. In PADs, very high values for the RC time (i.e., the time constant for the discharge of the pixel electrodes) can be achieved. These compositions likewise have reduced viscosity and enable AMDs to be operated at the first transmission minium, and exhibit no crystallization and no occurrence of smectic phases when stored in test cells of appropriate cell gap for times of at least 100 hours preferably 500 hours and most preferably 1000 hours at a temperature of xe2x88x9220xc2x0 C., preferably at xe2x88x9230xc2x0 C. and most preferably of xe2x88x9240xc2x0 C.
Preference is furthermore given to liquid-crystal mixtures which comprise at least one compound of the formula II 
in which
R3 is alkyl, alkenyl, alkyloxy or alkenyloxy having 1-8 carbon atoms,
Z3 and Z4, independently of one another, are a single bond, (CH2)2, OCH2, CH2O, COO, CN, F, CF2H or CF3 
X is OCF2H, OCF3, OCH2CF3, OCHFCF3, OCF2CF2H or F
L is H or F and
A4 is trans-1,4-cyclohexylene, 1,4-phenylene, 3-fluoro-1,4-phenylene or 3,5-difluoro-1,4-phenylene.
Particular preference is given to nematic liquid-crystal compositions which comprise at least one compound of the formula Ia 
in which
R1 and R2, independently of one another, are alkenyl or alkenyloxy having 1 to 8 carbon atoms,
and
Z2 is a single bond, xe2x80x94(CH2)2xe2x80x94 or trans-CHxe2x95x90CHxe2x80x94.
Z2 is particularly preferably a single bond.
Nematic liquid-crystal compositions of the present invention furthermore preferably comprise at least one compound of the formula III 
in which
R4 and R51 independently of one another,
are alkyl, alkoxy, alkenyl or alkenyloxy having 1 to 10 carbon atoms, in which 1 or a maximum of 2 non-adjacent CH2 groups may be replaced by O,
each Z5 if it is present a number of times independently of one another, is a single bond, COO, OCO, (CH2)2, OCH2, CH2O or trans-CHxe2x95x90CH,
each A5 and A6 independently of one another, and if A5 is present a number of times these radicals also independently of one another,
are trans-1,4-cyclohexylene, 1,4-cyclohexenylene, in which also one or two nonadjacent CH2 groups may be replaced by oxygen, 1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene, 2,6-difluoro-1,4-phenylene or 1,5-pyrimidinediyl, in which one or two H atoms are optionally replaced by F, and
n is 1, 2 or 3.
Very particular preference is given to nematic liquid-crystal compositions which comprise at least one compound of the formula Iaa 
in which
R1 and R2, are each, independently of one another, CHxe2x95x90CH2, CHxe2x95x90CHxe2x80x94CH2xe2x80x94CH3 and/or CHxe2x95x90CHxe2x80x94CH3 and
Z2 is (CH2)2 or a single bond.
Z2 is preferably a single bond and R1 and R2 are preferably identical to each other.
Preference is furthermore given to nematic liquid-crystal compositions which, besides compounds of the formula Iaa, comprise at least one further compound of the formula I 
and/or of the formula II 
In a preferred embodiment, the compositions according to the invention comprise from 10 to 34% of two or more compounds of the formula I.
The nematic compositions particularly preferably comprise at least two compounds of the formula Iaa from the group consisting of the formulae Ia1 to 1a9: 
The symmetrically substituted compounds of the formula Iaa, i.e., the compounds Ia1, Ia3, Ia5 and Ia8, are preferred, and of these, very particular preference is given to the 1-alkenyl compounds Ia1, Ia3 and Ia5. The best response times are achieved when using Ia1, Ia2 and Ia3. The most preferred compounds are those of the formulae Ia1 and Ia5.
The LC compositions according to the invention have a dielectric anisotropy, measured at 20xc2x0 C. and 1 kHz, of less than 4.0, preferably between 1.0 and 3.0, in particular from 1.5 to 2.5. The LC compositions according to the instant invention have a birefringence at 20xc2x0 C. and 589 nm of not more than 0.1, preferably not more than 0.08, especially preferred of not more than 0.075 and most preferred of below 0.07. The voltage holding ratio HR after 5 minutes at 100xc2x0 C. and IV using a commercial equipment of Antronic-Melchers, Germany, is at least 98%, preferably higher than 98.5%, especially preferred higher than 99.0% and most preferred at least 99.5%. The clearing point of the compositions according to the invention is above 750xc2x0 C., in particular above 80xc2x0 C., very particularly above 90xc2x0 C.
Compositions of this type preferably comprise two, three or four compounds of the formula I. Preferred compositions comprise at least of more than 10% by weight of the individual compounds of the formula I.
The compounds of the formula I are disclosed in the German patent applications and can be prepared analogously to known compounds.
The compounds of the formulae II and III are known to the person skilled in the art.
Preferred compositions likewise comprise two or more compounds of the formula III. Preference is given here to the compounds selected from the group consisting of the formulae IIIa and IIIb: 
in which R4 and R5 are as defined above under the formula III.
In a preferred embodiment, the compositions according to the invention comprise at least two compounds of the formula IIIa.
The compositions according to the invention preferably comprise from 6 to 48% by weight, in particular from 10 to 34% by weight or from 16 to 30% by weight, of components of the formula I. The percentages by weight for the other groups are preferably in the following ranges:
Formula II: from 10 to 45%, in particular from 18 to 38%
Formula III: from 5 to 52%, in particular from 12 to 40%.
The components of the formulae I, II and II preferably form the basis of the claimed compositions and together make up at least 60% by weight, particularly preferably at least 75% by weight, of the compositions However, it is also possible to use further LC components in smaller percentages in addition to the components of the formulae I to III for fine adjustment of the claimed compositions. In t his case, compounds having a high clearing point are used in particular.
In general, the compositions consist of a plurality of compounds mixed in a conventional manner. In general, the desired amount of the components used in the smaller amount are dissolved in the components making up the principal constituent, advantageously at elevated temperature. If the temperature selected is above the clearing point of the principal constituent, completion of the dissolution process is particularly easy to observe.
However, it is also possible to mix the components in a suitable solvent, for example in acetone, chloroform or methanol, and, after mixing, to remove the solvent again, for example by distillation under reduced pressure. It goes without saying that it must be ensured here that the solvent does not introduce any impurities or undesired doping substances.
By means of suitable additives, the liquid-crystal phases according to the invention can be modified in such a way that they can be used in any type of PAD which has been disclosed hitherto.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.